1. Field of the Invention
The present invention relates to a method of manufacturing hollow pistons such as, those used for compressors and a piston manufacturing machine.
2. Description of the Related Art
Weight reduction of a piston, which is to be used in a compressor driven by the reciprocating motion of the piston, is an important technical problem. Particularly in a swash plate compressor of a variable displacement type, the inertia force of a reciprocating piston caused by the reciprocating motion of the piston has a considerable influence on the inclination angle of the swash plate (that is, the control of discharge capacity). Therefore, in order to improve the control of the inclination angle of a swash plate, weight reduction of a piston, to reduce the piston inertia, is required.
Under these circumstances, various piston structures aiming at the weight reduction of a piston, used for a swash plate compressor, have been proposed. For example, a single-headed piston has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-105380, and Japanese Unexamined Patent Publication (Kokai) No. 11-107912. The single-headed piston has a hollow space inside the piston, which is inserted into each cylinder bore of a compressor, and a communication opening through which the hollow space communicates with the outside (a crank chamber, for example). By employing these structures, weight reduction of a piston is realized without a considerable decrease in strength of the mechanical structure of the whole piston. The piston is also designed so that lubricant oil (and a small quantity of refrigerant gas) is supplied from the cylinder bore to the crank chamber via the hollow space and the communication opening.
The piston of this type, however, has a problem in that oil adhered to the inner circumferential wall surface in the hollow space of the piston accumulates and it may remain inside the piston. Such accumulated oil may disadvantageously cause an increase in the weight of the piston and the hollow space cannot then achieve its purpose sufficiently.
With these above-mentioned problems being taken into account, the present invention has been developed. The purpose of the present invention is to provide a manufacturing method, and a piston manufacturing machine for a compressor, in which a hollow piston that can remain light while in use, after it is built into a compressor, can be manufactured efficiently.
To solve the above-mentioned problems, in the first aspect of the present invention, the method of manufacturing a piston for a compressor, in which a hollow piston is manufactured by welding plural piston parts, includes a process of placement in a reduced pressure area, in which a piston assembly is provided with a hollow space internally by assembling said plural piston parts and is placed in a reduced pressure area, and an electron beam welding process is applied to all the coupling portions of the piston assembly so that each of the above-mentioned piston parts are integrated into a unit under a reduced pressure, and the hollow space is formed into a hermetically sealed space the pressure of which is equal to that in the reduced pressure area.
According to the present invention, since the piston assembly is welded by an electron beam in a reduced pressure area, the hollow part (hollow space) in the piston assembly is sealed hermetically and contains a reduced pressure equal to that of a reduced pressure area. This means it is possible that the hollow space in the piston contains a reduced pressure by utilizing the pressure-reduced atmosphere for the electron beam welding. Moreover, lubricant oil does not seep into the hollow space because the hollow space is sealed hermetically. Therefore, the occurrence of accumulation of lubricant oil inside the piston can be avoided and the piston remains light during its reciprocating motion. In addition, because the hollow space of the piston contains little air, corrosion, by oxidation, at the surface of the piston hollow space can be suppressed.
The piston manufacturing machine of the second aspect of the present invention is equipped with an electron beam welding device which welds the coupling portion of the piston assembly by an electron beam with the pressure in the welding chamber being kept low, a conveying guide device attached to the electron beam welding device and equipped with a conveying guide, jigs, which have housing recesses that accommodate the piston assemblies and are positioned in the welding chamber of the electron beam welding device by the conveying guide device, a sealing means to isolate the housing recesses from the outside air when the jig is positioned in the welding chamber of the electron beam welding device and to respectively define the closed space for each housing recess, and a preliminary exhaust mean to preliminarily exhaust the closed space before the jig is positioned in the welding chamber.
According to the present invention, the jig, in which the piston assembly is accommodated in the housing recess, is conveyed to the welding chamber by the conveying guide device, and the closed space defined by a seal mean is preliminarily exhausted by the preliminary exhaust mean before the jig is positioned in the welding chamber. The electron beam welding device welds the piston assembly by electron beam in the reduced pressure area.
The present invention may be more fully understood from the description of the preferred embodiments of the invention set forth below, together with the accompanying drawings.